This invention relates to a method of for making blow-molded parts of thermoplastic material. The parts are formed from a hose-shaped parison after introducing it into a mold and closing the parison ends. The parison is expanded by pneumatic pressure for causing it to conform to the wall surfaces defining the mold cavity.
German Pat. No. 2,536,766 discloses a strip-shaped hollow body which is a closed, blow-molded part. Such blow-molded articles may be used as decorative or protective moldings. The parts are made by introducing a hose-shaped parison in a thermoplastic state into the mold, the ends of the parison are closed and the parison is expanded by inflation to thus assume the shape of the mold.
The inflation proper of the parison is effected by a hollow inflating needle or mandrel which pierces the parison while the mold is in a closed state. Air under pressure is introduced from the cavity of the blowing element into the inside of the parison and inflation pressure is generated therein which, as the wall thickness of the parison undergoes reduction, presses the parison wall against the inner walls of the mold cavity to thus produce the blow-molded part.
By means of a process outlined above, rigid blow-molded articles, such as decorative or protective moldings, bottles, and the like may be made. In the manufacture of bottles the hole through which inflation has taken place is enlarged to an opening of determined diameter by passing inflating or calibrating mandrels through the opening in the article.
In case such closed blow-molded bodies are, as semifinished products, submitted to further processing, it has been found to be disadvantageous that in addition to the making of the body other, separate process steps have been necessary, for example, to provide openings for mounting devices in the walls of the blow-molded body. Such additional process steps result in a significantly increased price of the final product.